Time domain vocoder

ABSTRACT

954,487. Telephone transmission systems. WESTERN ELECTRIC CO. Inc. April 28, 1960 [May 8, 1959], No. 14877/60. Heading H4R. Information relating to a message wave, e.g. a speech wave, is obtained by deriving from the message wave, a secondary wave coherent with the periodicities of the message wave but having a flat frequency spectrum, and determining the cross-correlation between these signals for various relative lags between the two waves. Signals representative of these correlations are transmitted over narrow band channels to enable synthesis of the message wave to take place at a receiver. In one embodiment the secondary wave is obtained by heavy clipping of the message wave. In a second embodiment the abovementioned clipper is succeeded by a differentiating circuit and a rectifier, so that the secondary wave consists of a pulse each time the message wave crosses the zero line in a particular direction. In a preferred form, however, the secondary wave consists of pulses coincident with peaks of the message wave. These pulses are preferably of amplitude proportional to the peak value of the message wave, or may take on more exaggerated values, effected by means of a volume expander. Thus, as shown in Fig. 3, the message signal is fed directly to a delay line 3 (terminated at 4 for no reflection) and also to a differentiator 11 followed by a clipper 12. The clipper output has a downward - going discontinuity corresponding to each positive peak of the message wave, and a pulse may be derived therefrom either by a differentiator circuit followed by a rectifier, or, as shown, by a monostable multivibrator 13. The output of circuit 13 may be fed direct to the multipliers 15 or preferably may be used to sample the message wave (expanded at 16) the output of the sampler 14 after being subjected to A.G.C. being taken for the secondary wave and fed to a series of multipliers 15, one of which is provided for each tapping of the delay line, and which serves to measure the correlation between the secondary wave and the output from the corresponding tap of the delay line. The outputs of the multipliers feed low-pass filters of narrow band-width, whose outputs #(#) are then transmitted. A pitch signal derived from an 80- 350 c.p.s. band filter is also transmitted. It is stated that in contrast with the earlier described methods, this last method results in a correlation function which is substantially symmetrical about #=0 (# representing the relative delays of the message wave and the secondary wave) and that therefore the number of tappings on the delay line may be halved as compared with the previous cases. In this case, in the receiver, Fig. 5, two signals require to be reconstructed from each one of the signals #(#) received. Thus the pitch signal after passing through a delay equalizer is clipped and each upward change of the clipped signal operates a mono-stable multivibrator 33 to produce an output pulse which is modulated in multipliers 35 by the respective signals #(#). The modulated pulses are fed to appropriate taps on a delay line 40, one of whose ends is open-circuited to provide an in-phase reflection. Thus for each input to a delay line tap # n , two pulses arrive at the load resistor 41, having delays # N Œ# n  respectively. The aggregate of these pulses pass through a low-pass filter 43 to produce the required message wave output. In the absence of a fundamental frequency relay 34 falls back to connect a noise source 36 to the multipliers. In this output the relative phases of the various components may be changed as compared with the original wave, but in the case of speech this has little effect on the intelligibility. In a modification, Fig. 4, the correlation is effected by feeding the signal to a high-loss attenuation pad 21 and thence to the delay line 3. Each tapping is then fed to summation circuit 22 and a difference circuit 23 whose other inputs are fed with the original signal. The outputs of these circuits are applied to full-wave rectifiers 24, 25 whose outputs are thus the absolute values of the sums and differences and the difference between these absolute values is taken at 26 and fed to a low-pass filter 27 whose output provides the appropriate one of the output signals #(#). The difference circuit 23 may be omitted, the circuit 26 taking the difference between the output of circuit 24 and the absolute value of the message signal. In all cases the message signal may be differentiated once or twice before being applied to the analysing apparatus, corresponding integrations being performed on the output from the synthesizer at the receiver. Specification 466,327 is referred to.

3 Sheets-Sheet 2 Filed May 8. 1959 /NVENT-OR M. RSCHROEDER )J f M70 M5"Jan. 1, 1963 M. R. scHRoEDER 3,071,652 y TIME DOMAIN vocoDER Filed Mays. 195e i 3 sheets-sheet s AMPL/T'UDE VUUV luv UV L P/rcH PER/oo 4.-P/rcH PER/oo /NVENrO/e' M. R. .SCHROEDER ByNvwy @Naf ATVORNEV UnitedStates Patent O 3,071,652 TIME DOMAlN VOCODER Manfred R. Schroeder,Murray Hill, NJ., assignor to Bell Telephone Laboratories, Incorporated,New York, N.Y., a corporation of New York Filed May 8, 1959, Ser. No.812,028 11 Claims. (Cl. 179-1555) This invention relates in general tothe modification of signals to facilitate their transmission, andparticularly to the reduction of their information rates and hence tothe compression of their frequency bands. Its principal specific objectis to compress the band of frequencies occupied by a telephone messagewave. A more general object is to apply new principles to the bandcompression or other modification of a message wave.

One -well known approach to the band compression problem, exemplified byDudley Patent 2,151,091, March 2l, 1939, is to divide the entirefrequency band occupied by a complex message Wave, eg., a speech wave,into a number of contiguous constituent subbands, to determine theenergy in each such subband, and to derive, for each subband, a controlsignal whose magnitude represents the subband energy. The analysis isperformed with a bank of filters to all of which the speech wave isapplied in parallel, while a rectifier connected to the output terminalof each filter delivers a signal representative of the energy passingthrough the lter. The resulting low frequency control signals, aftertransmission to a receiver station, control the synthesis of artificialspeech.

The analysis carried out by the apparatus of the Dudley patent isessentially an analysis according to Fouriers Theorem: It postulatesthat the speech wave may be represented, to whatever degree of precisionmay be required, by a harmonic series of components, each of which, byitself, is a pure sinusoid, and thus orthogonal to each of the others.To the extent that this postulate is untrue of the speech or othercomplex wave undergoing analysis, to that extent the control signalsderived by the apparatus fail accurately to represent the originalspeech wave, and the final synthetic speech fails to duplicate it.

A further difficulty with any frequency-domain approach such as that ofthe Dudley patent is inherent in the characteristics of the filters thateffect the breakdown of the voice into its constituent subbands. Bothfor physical realizability and to avoid excessive delays each of thesefilters must have a passband that is by no means negligible: it shouldoccupy a fraction of 1/10 to 1&0 of the entire speech frequency band.Now, while the responses of the several filters of such a group arecorrect and satisfactory when the several components of the appliedspeech wave are centered in the several filter passbands, phaseconsiderations make for unsatisfactory performance when the speech wavecomponents lic at or close to the crossover points between adjacentsubbands. Performance is still more unsatisfactory while eithercondition is in the course of changing to the other.

To avoid the difficulties inherent in the frequency-domain approach tovoice analysis, both those of principle and those of instrumentation,various proposals have been made to carry out the analysis in the timedomain without resort to filters. Both autocorrelation andcrosscorrelation techniques have 'been proposed. The autocorrelationtechnique suffers from the disadvantage that the autocorrelationfunction of a speech wave is inherently of a quadratic character.Unavoidably, this emphasizes components of large amplitudes at theexpense of components of smaller amplitudes and thus, unless thisquadratic character be removed, makes for distortion in thereconstructed speech. To remove it, however, presents seriousdifficulties.

ice

To escape the quadratic distortion imposed by the autocorrelationapproach it has already been proposed, notably by W. H. Huggins in ANote On Autocorrelation Analysis of Speech Sounds, published in theJournal of the Acoustical Society of America for` September 1954, vol.26, page 790, to carry out a cross-correlation -between the speech waveand a new wave consisting of pulses which have amplitudes that are allequal and independent of the original speech intensity but whose epochsare synchronized with the laryngeal pulses creating the original speech.Huggins suggests that the outcome of these operations will be a wavethat is free of the foregoing quadratic distortion. Such a wave mightthen be utilized to control the synthesis of artificial speech throughappropriate instrumentation not suggested by Huggins.

To derive the pulse train recommended by Huggins presents seriousdifficulties. It demands in principle that the commencement of eachperiod of the voice wave be unambiguously identified and that a pulse begenerated to mark its inception. This is a decision-making process whichit is by no means always possible to carry out with certainty; and ifthe pulses occur at the wrong instants on the time scale, the result isserious distortion in the reconstructed speech.

Accordingly, it is a principal object of the invention to analyze aspeech wave by carrying out a cross-correlation operation of the speechwave with a special reference wave that has two principal properties.First, and in order that the resulting cross-correlation signals shallbe first order counterparts of the voice wave in contrast to quadraticcounterparts, the reference wave is one of which the frequency spectrumis relatively flat; that is to say its variations with frequency aresmall compared with those of the spectrum of the speech wave itself.Second, and to avoid the decision-making process inherent in thegeneration of the pulse train of the Huggins monograph, a reference waveis chosen that is coherent with all, or at least most, of theperiodicities of the voice wave, notably the periodicities of itsprincipal formants and its fundamental or pitch period.

Many possibilities arise for a reference Wave having these properties. Afirst one is the clipped counterpart of the speech Wave itself or of aderivative of the speech wave. The clipped wave may be employed withoutfurther processing but, advantageously, the reference wave is itself thefirst derivative of this clipped wave, preferably after half waverectification. Another possibility is a train of samples of the speechwave, each taken at the instant at which a peak of the speech Waveoccurs. Still another possibility is a train of pulses of uniformamplitudes occurring at the successive peaks of the speech Wave, at itssuccessive zero crossings, or at successive points of the time scalethat mark some unambiguously identifiable property of the speech wave.

|Once a reference Wave having the required properties of spectraliiatness and coherence with the speech wave has been selected, it may begenerated or derived from the speech wave itself in any desired fashionby straightforward instrumentation in dependence on its otherproperties. Once the reference wave is thus generated, thecross-correlation operation between it and the original speech Wave maybe carried out in various Ways, several such being described below indetail. The result is a `group of control signals that are togetherrepresentative of the cross-correlation.

These control signals may betransmitted to a receiver station where,because they are representative of the speech wave to the first orderand without the quadratic distortion feature inherent in theautocorrelation analysis, they may be utilized directly in controllingthe synthesis of artificial speech. Because the individual controlsignals of this group vary only at syllabic rates the transmissionrequires a much narrower frequency band than does the transmission ofthe original speech.

The present invention envisions a further compression of the requiredtransmission band, and that is obtained in the following fashion. Thecontrol signals, taken together, determine all of the frequencycomponents required for the reconstruction of the original speech wavebut contain little information as to their phases. Now it has long beenan established fact that the human ear is entirely insensitive to smallphase shifts among the components which together make up a complex soundand, indeed, that large phase shifts, provided they are not excessive,do not affect intelligibility. Hence, an artificial wave having thecorrect frequency components but bearing little resemblance in form tothe original wave is indistinguishable, by the ear, from the originalwave itself. One such artificial wave is that in which the phases of theseveral components have been so shifted that the wave itself is asymmetrical one. Such symmetry means that the second half of the wave isa mirror image of the first half. Accordingly, the invention providesfor the development and transmission of the cross-correlation controlsignals for only one half of each speech wave period and for thegeneration, locally at the receiver station, of an artificial wave ofwhich the irst half is under control of the transmittedcross-correlation signals while the second half is generated withoutbenefit of additional transmitted information and simply by a repetitionof the rst half on an inverted time scale. This time scale inversion isconveniently carried out with the aid of a wave propagation device ordelay line that is terminated at one end for complete reflection withoutchange of phase.

The invention will be fully apprehended from the following detaileddescription of illustrative embodiments thereof taken in connection withthe appended drawings in which:

FIG. l is a graph of the absolute magnitude [xl of a quantity x plottedagainst its algebraic magnitude;

FIG. 2 is a schematic block diagram showing apparatus for analyzing aspeech wave to derive cross-correlation control signals;

FIG. 3 is a schematic block diagram showing apparatus alternative tothat of FIG. 2;

FIG. 4 is a schematic block diagram of apparatus for developingcross-correlation control signals in approximate form;

FIG. 5 is a schematic block diagram showing apparatus for synthesizing awave from received cross-correlation control signals;

FIG. 6 is a waveform diagram showing three consecutive periods of atypical speech wave;

FIG. 7 is a graph showing the cross-correlation function of a speechwave with a reference wave selected to endow the correlation functionwith even symmetry;

FIG. 8 is a diagram of assistance in explaining the operation of theapparatus of FIG. 5; and

FIG. 9 shows two consecutive periods of a synthetic wave developed bythe apparatus of FIG. 5.

Before entering upon a detailed description of 4the drawings it isdesirable to discuss certain mathematical relations, some of which areinstrumented by the apparatus shown in the drawings.

In corelation analysis a signal f(t) to be analyzed is compared, foreach of Ia number of different values of lag -r by which it is delayed,either with a reference signal or with an undelayed version of theoriginal signal. In particular, the cross-correlation p12 of two signalsf1.0) and f2(t) is given, -for any particular value of r, by

Autocorrelation on is similar, with the sole dierence 4 that one signalis a delayed version of the other. Thus,

1 t-l-T/z n f,f =ft T/2f t .fo-edt 2) In these expressions theintegration extends over an interval T symmetrically disposed about thetime t.

When the integration interval T is infinite, each of the foregoingexpressions is independent of the time t and depends on the lag -r only.When, to the contrary, the interval T is not infinite, each of theseexpressions depends on the time t as indicated. When the time dependenceis significant, the cross-correlation of (l) and the autocorrelation of(2) are of the so-called short term variety.

In what follows We shall deal with short term autocorrelation andcross-correlation, in contrast to the long term correlation functions.For simplicity of notation dependence of the correlation functions onthe time t is to be understood.

Further, for simplicity of notation and since each of the foregoingintegrals represents a time average, the simpler notation for such timeaverage, namely a superposed bar, will be employed.

With this understanding the autocorrelation given by Equation 2 becomes,for a message wave s(t),

This expression bears a quadratic relation to the wave s(t). Thisquadratic character can be emphasized by reformulating Equ-ation 3 inaccordance with the well known relation Thus,

The quadratic character of the autocorrelation function as given in (3)or (5) makes for certain diiculties in the instrumentation of thisfunction for wave analysis. Consider, however, two furthermodifications: First, the replacement of each of the two squared termsin the numerator of (5) by its absolute value. This leads to son(wherein the substitution of the symbol A, for the symbol p embraces allof these changes together, and indicates that it depends on themagnitude of e.

In the limit, as e approaches zero, the expression on the right remainsformally the same but the left-hand side is now independent of themagnitude of e. Thus,

Mulitiplying numerator and denominator of this expression by the delayedwave s(t-1) yields (To divide an expression containing a limit, by afinite quantity independent of the limit process, inside the limit signand to multiply by the same quantity outside the limit sign is wellestablished to be a justifiable procedure.)

The limit in the foregoing Expression 9 has the form of a differentialquotient. ln particular, it is the differential quotient of the function[xl with respect to x at the point z=s(t), for xeO. As is immediatelyapparent from FIG. 1, the differential quotient of` Ixl, for values of xother than zero, is simply the algebraic sign of x.

Reference to the Expression 9 itself shows that, for x=x(t)=0, theentire expression vanishes, thus taking account of the single caseexcepted above. Thus, defining the sign function.

-ll for x Sgn :c: Ofor 06:0 (10) -1 for x 0 (9) becomes }\(1)=s(t) sgns(t) (11) from which it appears that Mv) is in fact thecross-correlation of the wave s(t) with its own sign function. (It is,of course, of no importance which of the two factors is delayed withrespect to the other.)

But the sign function sgn s(t) of Ia speech wave s(t) is simply a formalrepresent-ation of the infinitely clipped counterpart of the same speechwave. Denoting the clipped counterpart of the wave s(t) by clp s(t),(11) becomes Inasmuch as the amplitudes of the clipped speech wave clps(r), `or of the sign wave sgn s(t) are always either +1, 0, or -1, itsonly significant features are its zeros and its spectrum. lts zeroscoincide, on the time scale,

.lith those of the original (undelayed) speech wave s(t).

Hence, it is fully coherent with all the periodicities of the speechWave, notably those of its fundamental and those of its variousformants. As for its spectrum, it is relatively flat on the frequencyscale; at any rate, much flatter than the spectrum of s(t) itself. Forthis reason Mt) as defined in (ll) or (l2) bears a first order spectralresemblance to s(t) and the restrictions imposed by the quadraticcharacter ofthe autocorrelation function of (3), (5), and (6) have beenescaped from. Moreover, it conrains all the information necessary forthe generation, after transmission to a receiver station, of anartificial wave that is fully as intelligible as the original speechwave and differs from the original speech wave in quality only to aminor extent.

It appears, therefore, that what is required, for the satisfactory timescale analysis of a speech wave, is to generate the cross-correlationfunction between the speech wave itself and a reference wave that (a) iscoherent with the speech wave, and (b) has a flat spectrum. The questionarises whether many waves lother than the clipped wave may not existwhich satisfy these4 requirements and which might, therefore, serveequally well as reference waves to be cross-correlated with the originalspeech Wave. lt turns out that there are many such. One such wave is thederivative of the clipped wave. This derivative wave has a positive peakfor each rising zero of the clipped speech wave and a negative peak foreach falling zero. It is thus coherent with the speech wave to preciselythe same extent as is the clipped Wave. The brief pulses of which it iscomposed make for a flat spectrum. At the price of a slight andunimportant reduction in coherence. its spectrum can be rendered flatterstill by employment of a half wave rectifier that blocks all thenegative pulses. Any reference wave -that satisfies the requirements ofspectral flatness and speech wave coherence may conveniently bedesignated c(t), Wherefore Equations 11 and l2 thus generalized, becomeAt this point it is desirable to distinguish between coarse flatness andfine flatness. The term coarse fiatness may be applied to the envelopeof a spectrum while fine atness refers to the density of its components.In the present example, blocking of all the negative pulses, as by ahalf wave rectifier, reduces the number of pulses occurring in each timeinterval by a factor 2, and this reduction in the time domain isreflected, in the frequency domain, as an increase in the number ofsignificant spec'- tral components. Thus, given the spectral componentsof the clipped Wave the spectrum of the half wave rectified pulse trainhas additional significant components. Thus the spectrum of the halfwave rectified pulse train has a greater degree of line scale flatnessthan does the spectrum of the clipped wave.

In the case of -the clipped speech wave itself and of its derivative,each of these reference waves is coherent with the original speech waveat the zeros of the latter. Presumably, coherence with the speech Waveat successive points of the time scale that mark any unambiguouslyidentifiable property of the speech Wave would serve equally well. 1thas been found that a reference wave having a flat spectrum and beingcoherent with the original speech Wave at the instants of its peaks,instead of its zeros, is equally ,suitable from the present standpointand more suitable from a different standpoint, to be discussed below. Atrain of pulses of like amplitudes, each identifying a positive peak ofthe speech wave, constitutes a reference wave of this character. It mayreadily be generated by first differentiating the speech wave, therebyto provide a derivative wave of which the zeros coincide on ythe timescale with the peaks of the original wave, clipping the derivative wave,differentiating the clipped wave and rectifying the differentiated wave.Still better, from the standpoint of fine scale tlatness, is to utilizeeach pulse of such a train to control the operation of a wave amplitudesampler. The output of the sampler is thus a train of pulses thatco-incide on the time scale with the peaks of the speech wave and ofwhich the amplitudes are proportional to the amplitudes of the speechwave at its successive peaks.

Still other reference waves having the required properties of spectralatness and speech wave coherence are possible, some of which will bediscussed below.

Referring now to the circuit diagrams, FIG. 2 showsy a system for the`development of a set of cross-correlation control signals by theinstrumentation of Equation ll or 12. A speech wave which may bederived, for eX- ample, from a microphone Lis first band-limited as by afilter 2 to meet the requirements of the sampling theorem. It is thenapplied, as a wave s(t) to the input point of a wave propagation deviceor delay line 3 of which the output point is terminated in a matchedimpedance element 4 to prevent reflection. The delay device, which maycomprise a plurality of like reactance networks connected in tandem,each having series inductance and shunt capacitance, is provided with aplurality of lateral taps that are numbered, in order, from o to n.Evidently, the wave s(t) reappears in succession at each of theselateral taps, and after a delay determined, in each case, by the lengthof the delay line 3 from its input point to that lateral tap asindicated on the drawing, for each tap, by the symbol -r with asubscript identifying the tap.

The energy paths extending from the oth tap and from the nth tap areshown in full, similar energy paths extending from the other taps ofthegroup being merely indicated. The signal appearing on the nth tap,having the waveform of the input signal s(t) but delayed byMrn, isevidently s(t-rn). This is applied to one input point of a modulator 5.

In a branch path the input wave s(t) is passed through an infiniteclipper 6 of any desired construction whose operation, as indicated byits input-output characteristic, is to reduce all positive amplitudes ofthe input wave to a uniform positive amplitude of -l-l and similarly -toreduce all negative amplitudes of the input wave to a uniform negativeamplitude of -1. The output o-f the clipper 6 is thus a clipped speechwave which, by comparison of Equation 11 with Equation 12 may bedesignated either sgn s(t) or clp s(t). With switches S1, S2 thrown tothe positions in which they are shown, the output of the clipper 6 isapplied directly to the other input point of the modulator 5.lntermodulation by the modulator of its two input signals results in thedevelopment of a complex modulation product wave which may berepresented as In the modulation process each frequency component of therst factor is multiplied by every frequency component of the secondfactor, components of sum and difference frequencies being thusdeveloped. The higher the frequency of the component, the more it tendsto be cancelled out, and the lower its frequency, the more it tends tobe preserved, in the time average. The entire complex of componentscontained in the product is now applied to a low pass filter 7 which, bypreserving the low frequency components of the product and blocking itshigh frequency components, operates to smooth it; i.e., to form its timeaverage, -thus to develop, on the nth output terminal of the apparatusthe wave of Equation 11, evaluated for 1:13,; i.e. \(rn). Similarly, bya like multiplication in a modulator 5', of the clipper output with theundelayed speech wave derived from the oth tap of the delay line 3, thesignal developed on the uppermost outgoing terminal is MTU).

With the switches S1 and S2 thrown to the positions indicated by thebroken lines the direct path is broken and the tandem combination of aditferentiator 8 and a half wave rectier 9 are inserted between theoutput point of the clipper 6 and the second input point of themodulator 5. This results in the multiplication of the delayed speechwave s(tr) by a reference wave c(t) consisting of a train of pulses oflike amplitudes, each coinciding with a positive-going zero crossing ofthe clipped speech wave. As explained above, the spectrum of such apulse train has greater fiatness, both coarse and fine, than does thespectrum of the clipped speech wave.

Provision of a similar modulator and low-pass filter for each of theremaining taps of the group and application of the output of the clipper6 to the second input points of all of the modulators results in thedevelopment of a set of cross-correlation control signals as indicatedat the right-hand portion of the drawing, one for each preselected valueof the lag T. en supplemented by a period control signal as describedbelow, they contain within themselves all of the information requiredfor the satisfactory reconstruction of a speech Wave that is, for allpractical purposes, indistinguishable from the original speech wave.

For synthesis of a satisfactory artificial speech wave Vfrom thesecross-correlation signals they must be supplemented with a pitch signalthat is indicative of the fundamental pitch of the speech wave. Certainadvantages are attained when, in addition, the pitch signal is coherentwith the fundamental periods of the speech wave. Apparatus fordeveloping a suitable noncoherent pitch signal is disclosed inapplication of G. Raisbeck, Serial No. 463,467, filed October 20, 1954,now matured into Patent 2,908,761, granted October 13, 1959. Apparatusfor developing a suitable period marker signal is disclosed in anapplication of B. P. Bogert and W. E. Kock, Serial No. 542,702, filedOctober 25, 1955, now matured into Patent 2,890,285 granted lune 9,1959, and also in an application of B. P. Bogert, Serial No. 578,097,filed April 13, 1956, now matured into Patent 2,928,901, granted March15, 1960. While apparatus of this kind is adequate in principle, itsometimes presents diiculties of instrumentation in practice. To avoidsuch difficulties, and at the price of a slight increase in the totalbandwidth of all the transmitted control signals it is preferred totransmit, without further processing, a narrow subband of the speechwave itself embracing the fundamental frequencies of all speech wavesthat may be encountered in practice. To this end a band-pass filter 10is provided, proportioned to pass only the lower portion of the speechrange, extending from cycles per second to 350 cycles per second. Theoutput of this filter 10 comprises a wave that is coherent with themajor peaks of the successive periods of the speech wave, and it serves,in cooperation with the cross-correlation control signals derived in thefashion described above, to control the synthesis of an artificialspeech wave.

The autocorrelation Vfunction of Equation 2 or Equation 3 is symmetricabout the origin of time. This symmetry reliects the fact that itcontains no information as to the phase relations among the componentsof the original speech wave from which it is derived.

ln contrast, the cross-correlation function of Equation 1 is in generalnot symmetric about the origin of time. The same is true of the modiedcross-correlation functions of Equations l1, l2 and 13. These can,however, be rendered approximately symmetric about the origin by aproper choice of the reference signal c(t); in particular, by choosingfor the reference function one whose pulses coincide in time with thepeaks of the speech wave in contrast to its zeros. With this choice theeffect of the multiplication and averaging operations called for byEquation 13 is roughly to pile the peaks, that occur in succession onthe t scale, one on top of another on the -r scale for 1=0, thus togenerate on the vscale a single peak that outweighs all the other peaksin amplitude. This results in imparting approximate symmetry to thecross-correlation function MT). This symmetry has important consequencesin making for further reductions in the frequency bandwidth or bit raterequired for transmission, as will be fully explained below.

FIG. 3 shows an alternative to the apparatus of FIG. 2 that incorporatesthe feature of peak coherence of the reference wave. A speech wavederived, for example, from a microphone is applied, after band-limitingby a filter, as a wave s(t) to the input point of a delay line 3 likethat of FIG. 2, similarly terminated in a matched impedance load 4 andsimilarly provided with a plurality of lateral taps that are similarlyidentied. The energy path from the nth lateral tap is shown in full,similar energy paths from the other taps of the group being merelyindicated. The nth tap extends to the first input point of a modulator15. The wave thus applied to this input point of the modulator isevidently .s0-rn).

A branch path from the microphone 1 extends to a differentiator 11 whoseoutput is applied to an infinite clipper 12 which may be identical withthe clipper 6 of FIG. 2. Because each positive peak of the band-limitedspeech wave s(tl is marked by a negative-going zero of its firstderivative the output of the clipper 12 is a rectangular wave that hasdownward-going discontinuities coincident with the positive peaks of thespeech wave and upward-going discontinuities coincident with itsnegative peaks.

In accordance with this aspect of the invention a reference wave c(t) isgenerated having a pulse for each downward-going discontinuity of theclipped wave and hence for each upgoing peak of the speech wave. Thegeneration of this reference wave may be instrumented in various ways,for example, with the aid of a diterentiator followed by a half waverectifier, poled to pass negative pulses and to block positive ones.Alternatively, a monostable or single trip multivibrator 13 may beprovided, adjusted to respond, by delivering output pulses of a singlepreassigned polarity, only to input pulses of negative polarity.

Evidently, merely by poling the half wave rectifier to pass positivepulses and to block negative ones and by correspondingly modifying thesingle trip multivibrator to respond only to pulses of positivepolarity, the output of this unit would comprise a train of pulses eachof which is coincident on the time scale with one of the down-goingpeaks ofthe speech Wave. Because the speech wave is not, in general,entirely symmetric about the time axis, the decay of the envelope of itsup-going peaks` may differ from the corresponding decay of the envelopeof its down-going peaks. It is preferred to derive the pulse train fromthat set of speech wave peaks, up-going or down-going, that has thegreater envelope decay.

It is also remarked, in passing, that the terms upgoing and down-goingrefer to the graphic representation of the speech wave, so that thecorrespondence between the polarities on the graph and the differentialair pressures at the mouth of the speaker that they represent isarbitrary.

The Output of this element, c1(t), may if desired be utilized directlyand, provided the switch S3 is closed, applied to the second input pointof the modulator 15. For the sake of gaining the additional finefiatness provided by amplitude variations among these pulses it ispreferred to employ them as control pulses to operate a sampler 14 whichthus delivers, for each such pulse, an amplitude sample of the speechwave applied to its input conduction terminal. The sequence of suchsamples, designated 02(1), is now applied, provided the switch S3 isopen, to the second input point of the modulator 15.

As a refinement, and to accentuate the improvements that result fromnonuniform amplitudes of the pulses of the reference wave, the speechwave may be predistorted before application to the conduction terminalof the sampler. The predistorting element 16, which expands theamplitude scale of the speech wave may, for example, have aninput-output characteristic that obeys any odd power law; for example,the output may be proportional to the cube of the input.

In the event that this amplitude expansion is employed at the inputterminal of the sampler, to avoid overloading the modulator 15 and inorder to avoid distortion of the ultimately reconstructed speech, anautomatic gain control device 13 having a suitably long time constant ispreferably included.

The output of the modulator 15 is passed through a low-pass filter 17which operates to smooth or average it, thus to develop on the nthoutput terminal of the apparatus the correlation function of Equation 13evaluated for 1=1n. Provision of a similar modulator and lowpass filterfor each of the remaining taps of the group, and application of thereference wave C?) or c2(t) to the second input points of all of themodulators results in the development of a set of cross-correlationcontrol signals as indicated at the right-hand portion of the drawing,one for each preselectedI value of the lag f. inasmuch as each of thesecontrol signals satisfies the requirements of spectral fiatness andcoherence, the same requirements are satisfied by the control signalstaken as a group; and when as described in connection with FIG. 2 theyare supplemented by a period control signal provided by a basebandfilter 10, they contain within themselves all of the informationrequired for the synthesis of a satisfactory artificial speech wave thatis, for all practical purposes, indistinguishable from the originalspeech wave.

Returning to Equation 7, it will be recalled that the cross-correlationfunctions of Equations 11, 12 and 13 Wave.

10 were developed by passing to the limit at'which e approaches zero;Le., by going from equation 7 to Equation 8.

Valuable results, however, may be secured without passing to this limit,by vholding e to a magnitude that is a small quantity but by no means aninfinitesimal one; i.e., by the instrumentation of Equation 7. It may,for example, have the value Y10. FIG. 4 shows apparatus by which theapproximate correlation function 7 is instrumented. With the switches S4and S5 closed a speech wave originating, for example, in a microphone 1passes, after band-limiting by a filter 2, through an attenuator 21which reduces its amplitude from s(t) to es(t). The speech wave thusattenuated is applied to a delay line 3 like that of FIGS. 2 and 3 andsimilarly terminated in a matched impedance load 4 and provided with anumber of lateral taps designated in the same fashion as those of FIGS.2 and 3. The energy path extending from the nth tap is shown in full,the others being understood. The wave appearing on the nth tap isevidently es(t1n). This delayed and attenuated wave is added to andsubtracted from the original speech wave by an adder 22 and a subtractor23. The resulting sum and difference waves are converted to theirabsolute magnitude counterparts by full wave rectifiers 24, 25 and afinal subtractor 26 forms the difference between the outputs of the tworectiers. Reference to Equation 7 shows immediately that the output ofthis final subtractor is equal to ZexEh-n). As above remarked, thefactor 2e is merely a scale factor. Since it is common to all of thecorrelation function outputs it can be disregarded, or compensated by anamplifier, as preferred. The resulting signal, after smoothing by afilter 27, passes to the nth output terminal of the apparatus.

Without significant change in the character of the wave represented byEquation 7, the fourth term may be omitted. This omission, however,reduces the magnitude of the numerator by a factor 2. Accordingly, topreserve the magnitude of the fraction, the denominator may likewise bereduced by a factor 2. These two alterations together lead to It will benoted that Equations 7 and 14 follow the two standard forms for a finitedifference quotient.

This change from Equation 7 to Equation 14 results in certain obvioussimplifications of instrumentation in FIG. 4.

The set of control signals appearing on the several conductors at theright-hand portion of FIG. 4, whether this figure be constructed toinstrument Equation 7 or Equation 14, thus constitute an approximaterepresentation of the cross-correlation function of Equation ll. As inthe case of the other figures, they are supplemented by a period controlsignal which may be developed by a baseband filter 1f?. i

If preferred, the operations of the apparatus of FIG. 4 may be appliedto the first derivative of an incoming speech wave or to its secondderivative, instead of to the unaltered speech wave itself. To this end,opening of either one of the switches S4, S5, introduces a singledifferentiator, 28 or 29, into the path from the microphone to the delayline 3, and so applies the first derivative of the speech Wave to theapparatus and opening both switches together introduces both thedifferentiators 28, 29 and so applies the second derivative of thespeech wave to the apparatus. The resulting control signals aretherefore representative of the approximate cross-correlation, with asuitable reference wave, of the first or the second derivative of thespeech wave, as the case may be. After transmission of these controlsignals to a receiver station an artificial wave may be synthesizedwhich refiects the significant features of this first or secondderivative of the The synthesized wave may then be integrated,

once for the first derivative or twice for the second derivative, torecover a synthetic wave having the same spectral character as thespeech wave.

The same refinement may be introduced in the apparatus of FIG. l or thatof FIG 3. To avoid complexity of drawings the circuit details have notbeen shown.

The cross-correlation control signals and the accompanying periodcontrol signal are now to be transmitted to a receiver station, there tocontrol the synthesis of an artificial speech wave. If they arerepresentative of an asymmetric cross-correlation function they shoulddo so for each full speech wave period. (Three consecutive periods of atypical speech wave are shown in FIG. 6.) In this event the synthesizingapparatus may be of known character, for example, as described in theaforementioned application of W. E. Kock and B. P. Bogert. To cover thefull speech wave period with suicient detail for high quality of thesynthetic speech wave requires a` considerable number of such controlsignals.V Thus, for example, the fundamental or pitch frequency of abass voice may be as low as 100 cycles per second and this means afundamental period of 10 milliseconds duration. For adequatereproduction of the harmonic and formant frequencies of such a voice upto and including 3000 cycles per second, the Nyquist sampling rate is6000 samples per second, which gives a Nyquist period of 1/6millisecond. The ratio, then, of the fundamental speech wave period tothe Nyquist interval is l ms. its ms.

As indicated above, however, proper choice of the reference wave c(t),especially choice of a wave that is coherent with the peaks of thespeech wave in contrast to its zeros, makes for symmetry of thecross-correlation function about the origin. Such a cross-correlationfunction, having even symmetry about the origin, is shown in FlG. 7,where the dots on the right-hand branch represent its discrete values ascarried by the control signal channels; i.e., M), M11) MTB), etc. Inaccordance with one aspect of the invention this symmetry is turned toaccount in a fashion which permits the reduction of the number ofindividual control signal channel by a factor 2, i.e., in the presentexample a reduction from 60 such channels to 30 such channels. Broadlyspeaking, the manner in which this result is achieved is as follows:

'The cross-correlation being symmetric about the origin it may bereproduced under control of the transmitted control signals, for onehalf of a fundamental speech wave period and, by a scanning process,converted into a wave in the time domain. The wave thus generated is nowreproduced again on an inverted time scale after a delay of one half ofthe speech wave period, and without benefit of any additionaltransmitted information. The result of this step is to generate a timewave which is a mirror image of the rst time Wave and occupies thesecond half of the speech wave period. The result of these two steps,taken together, is a symmetrical time wave that occupies the full speechwave period. This wave is coherent with the original speech wave and hasthe same spectrum. Hence its intelligibility is no less than that of theoriginal speech wave and its quality differs only slightly. When plottedas a graph its appearance may differ widely from that of the originalspeech wave which is normally far from symmetrical. This wide differencein appearance is a consequence of the suppression, in the synthesizedwave, of all information as to the relative phases of the variouscomponents of which the original speech wave is constituted. Since it isa well established fact that the ear is largely insensitive to suchphase shifts, at least within a single fundamental period, they are ofno moment the reproduction. Thus, the improvement in econonfg." by afactor 2 that results from the use of the symmetric cross-correlationfunction is purchased at the price of only a negligible amount ofdegradation of the quality of the synthetic speech.

FIG. 5 shows apparatus by which the foregoing scheme is instrumented.The incoming baseband signal, after first passing through a delayequalizer 30, is applied to the input terminal of an energy source 31which may comprise a clipper 32 and a single trip multivibrator 33connected in tandem. The operation of this apparatus is to deliver atrain of pulses, one for each up-going Zero of the clipped basebandwave. Provided the baseband pitch signal is above a preassignedamplitude threshold it energizes a relay winding 34 thus to hold thetongue of the relay against its front fixed contact so that the pulsesdelivered by the single trip multivibrator 33 are applied to one inputpoint of a modulator 35. When, as in the case of unvoiced speech thebaseband signal fails, the relay tongue falls to the back contact, thusto deliver to the modulator 35 the output of a noise source 36.

The `several cross-correlation signals, after reaching the receiverstation, appear on the several conductors shown at the left-hand marginof the ligure. Each one is identified by the cross-correlation signalthat it carries, and these are numbered in order from MTG) to MTN).Hence the signals that these conductors carry individually areproportional to the yamplitudes of the dots in FIG. 7, lso that Ithesignals on all the conductors taken as a group constitute a spacepattern that is at every point proportional to the right-hand portion ofthe cross-correlation curve of FIG. 7.

The energy paths from the third one of the incoming conductors and fromthe nth one are shown in full. The others are to tbe understood assimilar. Thus, the signal Mrn) is applied to the second input point ofthe nth modulator 35 of which the output terminal is connected to alateral tap, designated 1n, of a delay line 40 having lateral tapsnumbered in order from O to N. This delay line 40 may be of the sameconstruction as those shown in the other figures and is similarlyterminated at its right-hand end in a matched impedance load 41. Unlikethe other delay lines, however, the delay line 40 of FIG. 5 isterminated at its left-hand end for complete reection. When, as with thereference signal employed in FIG. 3, the symmetry of thecross-correlation function about the origin is even (FlG. 7), thereflection should be without change of phase, as by an open circuit asshown in FIG. 5. In contrast, in any case in which the cross-correlationfunction has odd symmetry about the origin, the reflection shouldinclude a phase inversion, as Iby a short circuit.

With this arrangement the pulse output of the nth modulator 35, afterentering the delay line 40 at the nth lateral tap, travels in bothdirections Traveling to the right it reaches the load 41 directly aftera delay of rN-q-n. Traveling to the left it reaches the open circuitterminals after a delay of Tn whereupon it is reflected, travels to theright, and reaches the load 41 after a further delay of TN. Thus eachpulse entering the delay line `at the nth tap is reproduced in the loadtwice: once after a delay lrN-Tn and again after a delay TN-f-rn. AS aresult, for each pulse reproduced earlier than t=TN, a mirror image ofthis pulse is reproduced `at a later time, symmetrically located on thetime scale with respect to the time tzrN. The fixed delay 1N is of noconsequence.

FIG. 8 shows such a. pulse pair for 1=rn and another such pulse pair for7:73. The two members of each pair are symmetrically disposed on thetime scale about the point t=TN. In conformity with FIG. 7, the twopulses of the pair for 1-:1-3 are indicated as being of larger amplitudethan the members of the pulse pair for 7:71,. The two selected values r3and 7 are only representative of the number indicated in FIG. 7 by thedots on the correlation function curve. A complete representation in thefashion of FIG. 8 would include a pulse pair for each such dot, the twomembers of each pair 13 being of like amplitudes and symmetricallydisposed about the point t=rN and would have magnitudes and polaritiesin proportion to the magntudes and polarities of the dots in FIG. 7.

The operation of the reflecting delay line 40 is thus to `scan the groupof incoming cross-correlation conductors, commencing at the highestnumbered one and proceeding in one direction to the lowest numbered one,and then immediately -to scan them again in the opposite direction fromthe lowest numbered one to the highest numbered one, thus to reproducethe cross-correlation appearing on each single one of these conductorsas a pair of pulses. All of the pulses picked olf the incomingconductors in the course of the first scan constitute a first time waveportion, and this is immediately followed by a second time wave portion,constituted of all of the pulses picked off in the course of the secondscan. These two time wave portions constituted, as they are, of thepulses picked off in the cour'se of the two successive scans, are now'smoothed as by a low-pass filter 43 proportioned to have its cutofffrequency at about 3000 cycles per second. The pulse train as thussmoothed appears as in FIG. 9 and may now be applied directly to areproducer 44 which delivers intelligible and natural sounding speech;and this despite the disparity in appearance between the synthetic waveof FIG. 9 and the original speech wave of FIG. 6. As above explained,this disparity in appearance results from the suppression, in thecross-correlation control signals, of all information designating thephase positions, within a fundamental speech wave period, of the severalfrequency components of which it is constituted. As above indicated,this phase information is of minimal importance to the ear.

While lthe invention has been described las applied to a speech wave, itwill be readily apparent to those skilled in the lart that it is ofgeneral application to message waves provided only that their statisticsare such that the cross-correlation of the message wave with -a suitablereference wave that has a relatively flat spectrum and is coherent withthe message wave is a meaningful quantity.

What is claimed is:

1. Apparatus for analyzing a message wave and for developing controlsignals for use in the reconstitution of an artificial message wavewhich comprises means for 4 generating, under control of said messagewave, an auX- iliary wave having a relatively flat frequency spectrumand being coherent with at least several of the periodicities of themessage wave, means for delaying one of said message and auxiliary waveswith respect `to the `other by each of a plurality of different lags,modulator means for developing, for each of said lags, a modulationproduct wave of said delayed wave by 'said undelayed wave, and filtermeans for smoothing said modul-ation product waves to provide a group ofcontrol signals that are together representative of the `significantcharacteristics of said message wave. Y

2. Apparatus for analyzing a message wave and for developing controlsignals for use in the reconstitution of an artificial message Wavewhich comprises means for generating, under control of said messagewave, an auxiliary wave consisting of a train of pulses of alternatelyopposite polarities and coincident in time with all of the severalzeroes of said message wave, a rectifier having an input terminal and anoutput terminal, said input terminal being coupled in tandem with saidgenerating means, said rectifier thus acting to eliminate from saidtrain all of the pulses of said train that are of one polarity withoutaffecting the pulses that are of the other polarity, whereby said trainas thus modified is characterized by a relatively flat frequencyspectrum and by coherence with the several periodicities of the messagewave, means for delaying said message Wave with respect to said modifiedtrain by each of a plurality of different lags, a plurality ofmodulators, one for each of said lags, each modulator having two inputterminals and an output terminal, connections extending from the outputterminal of said rectifier to one terminal of each of said modulators,connections for applying the variously delayed message Waves to theother input terminals of said modulators, one to each, said modulatorsthus acting to develop, for each of said lags, a modulation product waveof the delayed message wave by the modified pulse train, and filtermeans for smoothing said modulation product waves to provide a group ofcontrol signals that are together representative of the significantcharacteristics of said message wave.

3. Apparatus for analyzing a message wave characterized by up-goingpeaks alternating with down-going peaks and for developing controlsignals for use in the reconstitution of an artificial message Wavewhich comprises means for generating, under control of said messagewave, an auxiliary wave consisting of a train of pulses that arecoincident in time with selected ones of the successive peaks of saidmessage wave, whereby said train is characterized by a relatively flatfrequency spectrum and 'oy coherence With at least several of theperiodicities of the message wave, means for delaying one of saidmessage and auxiliary waves with respect to the other by each of aplurality of different lags, modulator means for developing, for each ofsaid lags, a modulation product wave of said delayed wave by saidundelayed wave, and filter means for smoothing said modulation productwaves to provide a group of control signals that are togetherrepresentative of the significant characteristics of said message wave.

4. Apparatus for analyzing a message wave characterized by up-goingpeaks alternating with down-going peaks and for developing controlsignals for use in the reconstitution of an artificial message Wavewhich comprises means for generating, under control of said messagewave, a train of pulses that are coincident in time with the successivepeaks of said message Wave that extend in one direction from the messagewave axis, means for sampling the amplitudes of said message wave undercontrol of the pulses of said train to provide an auxiliary waveconsisting of a peak sample train, whereby said peak sample train ischaracterized by a relatively flat frequency spectrum and by coherencewith the periodicities of the message wave, means for delaying one ofsaid message and auxiliary waves with respect to the other by each of aplurality of different lags, modulator means for developing, for each ofsaid lags, a modulation product wave of said delayed Wave by saidundelayed Wave, and filter means for smoothing said modulation productWaves to provide a group of control signals that are togetherrepresentative of the significant characteristics of said message wave.

5. Apparatus for analyzing a message wave characterized by 11p-goingpeaks alternating with down-going peaks and for developing controlsignals for use in the reconstitution of an artificial message wavewhich comprises means for generating, under control of said messageWave, a train of pulses that are coincident in time with the successivepeaks of said message wave that extend in one direction from the messagewave axis, means for expanding the amplitude scale of said message wave,means for sampling the expanded amplitudes of said message wave undercontrol of the pulses of said train to provide an auxiliary waveconsisting of said expanded peak sample train, whereby said auxiliarywave is characterized by a relatively flat frequency spectrum and bycoherence with the periodicities of the message wave, means for delayingone of said message and auxiliary Waves With respect to the other byeach of a plurality of different lags, modulator means for developing,for each of said lags, a modulation product wave of said delayed wave bysaid undelayed wave, and filter means for smoothing said modulationproduct waves to provide a group of control signals that are togetherrepresentative of the significant characteristics of said message wave.

6. Apparatus for synthesizing an artificial wave from a plurality ofincoming control signals that are together representative of thecross-correlation function of an original message wave with a referencewave which comprises means for presenting said signals as an extendedspace pattern, means for scanning said space pattern from end to end inone direction to generate a first time wave portion, means forimmediately re-scanning said space pattern from end to end in theopposite direction to generate a second time wave portion that iscontinuous with said first time wave portion, and means for reproducingsaid first and second time wave portions in sequence.

7. Apparatus for synthesizing an artificial wave from a plurality ofincoming control signals that are together representative of thecross-correlation function between an original message wave and areference wave, each of said control signals being individuallyrepresentative of said cross-correlation for a single preassigned valueof a lag T, said values of 1- increasing monotonically from the valuezero for the first of said control signals to the value TN for the lastof said control signals, which comprises an elongated wave propagationdevice having a first end terminated in a matched impedance load and asecond end terminated for complete reflection, said device beingprovided with a plurality of lateral taps equal in number to saidincoming control signals, means under control of an incoming pitchsignal for generating a train of pulses that are coherent with saidoriginal message wave, a plurality of modulators equal in number to saidincoming control signals, each of said modulators having two inputpoints and an output point, means for applying said several incomingcontrol signals to the rst input points of said several modulators, oneto each, means for applying said pulse train to the second input pointsof all of said modulators, connections from the output points of theseveral modulators to the several taps of said propagation device, oneto each, and means for reproducing the wave appearing in said loadimpedance.

8. Apparatus for analyzing a message wave comprising consecutivefundamental periods each of which is divisible into a first half and asecond half period to develop narrow band control signals, and forsynthesizing an articial message wave from said control signals, whichcornprises means for generating, under control of said message wave, anauxiliary -wave consisting of a train of pulses that are coincident intime with peaks of said message wave, whereby said train ischaracterized by a relatively flat frequency spectrum and by coherencewith the several periodicities of the message wave, and the crosscorrelation of said train with an entire message Wave period ischaracterized by even symmetry, means for delaying one of said messageand auxiliary waves with respect to the other by each of a plurality ofdifferent lags that together span the rst half, only, of eachfundamental message wave period, modulator means for developing, foreach of said lags, a modulation product wave of said delayed Wave bysaid undelayed wave, lter means for smoothing said modulation productwaves to provide a group of control signals that are togetherrepresentative of the correlation between the first halves of themessage wave periods and the auxiliary wave, means for transmitting saidcontrol signals to a receiver station and, at said receiver station,means for reconstructing from said control signals a replica, for eachmessage wave period, of said first half period correlation, means forthereupon generating an image, of reversed time-order, of said firsthalf period correlation, and means for reproducing said first halfperiod correlation and said image in immediate succession.

9. Apparatus for analyzing a message wave comprising consecutivefundamental periods each of which is divisible into a first and a secondhalf period to develop narrow band control signals which comprises meansfor generating, under control of said message wave, an auxiliary waveconsisting of a train of pulses that are coincident in time with peaksof said message wave, whereby said train is characterized by arelatively fiat frequency spectrum and by coherence with the severalperiodicities of the message wave, and the cross correlation of saidtrain with an entire message Wave period is characterized by evensymmetry, means for delaying one of said message and auxiliary waveswith respect to the other by each of a plurality of different lags thattogether span the first half, only, of each fundamental message waveperiod, modulator means for developing, for each of said lags, amodulation product wave of said delayed wave by said undelayed wave,filter means for smoothing said modulation product waves to provide agroup of control signals that are together representative of thecorrelation between the first halves of the message wave periods and theauxiliary wave, means for transmitting said control signals to areceiver station and, at said receiver station, means for synthesizingan artificial message wave from said control signals.

10. Apparatus for analyzing a periodic message wave to develop narrowband control signals and for synthesizing an artificial message wavefrom said control signals, which comprises means for developing, fromeach period of said message Wave, a control signal constituted of afirst half and a second half and having even symmetry, means fortransmitting to a receiver station the first half, only, of each controlsignal and, at said receiver station, means for reconstructing from saidcontrol signals a replica, for each message wave period, of said firsthalf control signal, means for thereupon generating an image, on aninverted time scale, of said first half control signal, and means forreproducing said first half control signal and said image in immediatesuccession.

11. In a system comprising a transmitter station and a receiver station,transmitter station apparatus for analyzing a message wave comprisingconsecutive fundamental periods each of which is divisible into a firstand a second half period to develop narrow band control signals whichcomprises means for generating, under control of said message wave, anauxiliary wave consisting of a train of pulses that are coincident intime with peaks of said message wave, whereby said train ischaracterized by a relatively at frequency spectrum and by coherencewith the several periodicities of the message Wave, and the crosscorrelation of said train with an entire message wave. period ischaracterized by even symmetry, means for delaying one of said messageand auxiliary waves with respect to the other by each of a plurality ofdifferent lags that together span the first half, only, of eachfundamental message wave period, modulator means for developing, foreach of said lags, a modulation product wave of said delayed wave bysaid undelayed wave, filter means for smoothing said modulation productwaves to provide a group of control signals that are togetherrepresentative of the correlation between the first halves of themessage wave periods and the auxiliary wave, means for transmitting saidcontrol signals to a receiver station and, at said receiver station,means for synthesizing an artificial message Wave from said controlsignals which comprises an elongated wave propagation device having afirst end terminated in a matched impedance load and a second endterminated for complete reliection, said device being provided with aplurality of lateral taps equal in number to said incoming controlsignals, means under control of an incoming pitch signal for generatinga train of pulses that are coherent with said original message wave, aplurality of modulators equal in number to said incoming controlsignals, each of said modulators having two input points and an outputpoint, means for applying said several incoming control signals to thefirst input points of said several modulators, one to each, means forapplying said locally generated pulse train to the second input pointsof all of said modulators, connections from the output points of theseveral modulators to the several taps of said propaga- References Citedin the le of this patent UNITED STATES PATENTS Dudley Mar. 19, 1940Oliver Ian. 24, 1956 Dudley et al. Nov. 20, 1956 lFeldman et a1. Nov. 4,1958 Bogart et al. June 9, 1959 Edson et a1 Sept. 29, 1959 Bogert Mar.15, 1960

1. APPARATUS FOR ANALYZING A MESSAGE WAVE AND FOR DEVELOPING CONTROLSIGNALS FOR USE IN THE RECONSTITUTION OF AN ARTIFICIAL MESSAGE WAVEWHICH COMPRISES MEANS FOR GENERATING, UNDER CONTROL OF SAID MESSAGEWAVE, AN AUXILIARY WAVE HAVING A RELATIVELY FLAT FREQUENCY SPECTRUM ANDBEING COHERENT WITH AT LEAST SEVERAL OF THE PERIODICITIES OF THE MESSAGEWAVE, MEANS FOR DELAYING ONE OF SAID MESSAGE AND AUXILIARY WAVES WITHRESPECT TO THE OTHER BY EACH OF A PLURALITY OF DIFFERENT LAGS, MODULATORMEANS FOR DEVELOPING, FOR EACH OF SAID LAGS, A MODULATION PRODUCT WAVEOF SAID DELAYED WAVE BY SAID UNDELAYED WAVE, AND FILTER MEANS FORSMOOTHING SAID MODULATION PRODUCT WAVES TO PROVIDE A GROUP OF CONTROLSIGNALS THAT ARE TOGETHER REPRESENTATIVE OF THE SIGNIFICANTCHARACTERISTICS OF SAID MESSAGE WAVE.